Introduction

Global warming will continue being a critical problem in the twenty-first century due to increased carbon dioxide and other greenhouse gases (GHGs) concentration in the atmosphere. The atmospheric concentrations of the greenhouse gases carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) is increasing from time to time. In 2011 the concentrations of these GHGs were 391 ppm, 1803 ppb, and 324 ppb, which exceeded the pre-industrial levels by about 40%, 150%, and 20%, respectively (IPCC, 2013).

The exchange of carbon (C) between soils and the atmosphere is a significant part of the carbon cycle since carbon is a major component of soils as organic matter content. Globally, the soil organic carbon (SOC) pool stores ~ 1500 PgC in the first one meter depth of soil, which is more carbon than is contained in the atmosphere (~ 800 PgC) and terrestrial vegetation (~ 500 PgC) (FAO and ITPS, 2015) although their distribution is spatially and temporally variable. The 2300 Gt of C stored in global soil is 3 times the size of the atmospheric C pool and 4.1 times the biotic C pool (Lal, 2003).

SOC is the main component of soil organic matter (SOM). According to FAO and ITPS (2015), SOM contains roughly 55–60% C by mass. A high SOM content provides nutrients to plants and improves water availability, which enhances soil fertility and improves crop productivity. Moreover, SOC improves soil structural stability and porosity, which ensures sufficient aeration and water infiltration to support plant growth.

The carbon flux derived from land use change was more predominant in preindustrial times (between 1750 and 2011) in which one-third of all anthropogenic GHGs was derived from land use changes (IPCC, 2014). On a long-term basis, atmospheric CO2 has increased from about 180 to 280 ppm since the last glacial period, adding about 220 PgC to the atmosphere over a 10,000 year period at a rate of ~ 4.4 PgC yr.− 1 (Baldocchi et al., 2016).

The global soil water erosion process has been described both as a net C source of around 1 Gt yr.− 1 (Lal, 2003) and a net C sink of up to 1.5 Gt yr.− 1 (Stallard, 1998). The carbon-based GHGs emitted by soil are CO2 and methane (CH4), which are two of the most leading anthropogenically emitted GHGs (IPCC, 2014). This means dead organic material incorporated into the soil can be potential sources of carbon through organic material transformation by heterotrophic microorganisms. SOC sequestration is the process by which carbon is fixed from the atmosphere via plants or organic residues and stored in the soil. When dealing with CO2, SOC sequestration involves three stages: (i) the removal of CO2 from the atmosphere via plant photosynthesis; (i) the transfer of carbon from CO2 to plant biomass; and (i) the transfer of carbon from plant biomass to the soil where it is stored in the form of SOC (FAO, 2017).

As a result of soil erosion & deposition processes, the sediments trapped by sediment trapping dams or structures can potentially serve as natural carbon sinks (Poch et al. 2006; Harden et al. 2008; Van Oost et al. 2008; Cao et al. 2009, 2010). The organic carbon content in deposited sediments & eroded material was over twice that of the original soils (Jacinthe et al., 2004). Globally sedimentation resulting from soil erosion can sequester ~ 1 Pg C yr.− 1 (Stallard 1998; Smith et al. 2001). Dean and Gorham (1998) estimated the organic carbon sequestering role of reservoir sediments with a rate of 0.16 × 1015 g yr.− 1. Floodplain sediments have also been documented as potential organic carbon stores up to 0.22 kg C m− 2 yr.− 1 (DeLaune and White, 2012; Kayranli et al., 2010).

SOC sequestration for check-dams (CDs) was evaluated in China (Bao, 2008; Wang et al. 2011) based on the sediments volume & average SOC contents. The result showed that CDs trapped over 21*109 m3 with an average SOC content of ~ 3.31 g kg− 1 (Bao, 2008) and 0.952 Gt (1Gt = 109 t = 1015 g) with an average SOC content of 3.4 g kg− 1 (Wang et al., 2011).

In Ethiopia, CDs and sediment storage dams (SSDs) have been implemented in large areas as sediment trapping measures, which stored large amounts of sediment (Mekonnen et al., 2015; MERET, 2008). Together with the sediment, these structures are trapping and storing large amount of organic carbon. However, the amount of carbon sequestered together with the sediment and their climate change mitigation role is not yet evaluated (MERET, 2008). According to Li et al. (2007) and Cao (2008), although CDs, which are widely used to trap sediments in areas with high soil erosion, are acting as carbon sinks, only a few assessments of their carbon sequestration have been performed.

Therefore, the objectives of this study were to (i) quantify the amount of sediment trapped and stored by check dams (CDs) and sediment storage dams (SSDs) in northwest Ethiopia, (ii) determine the amount of soil organic carbon (SOC) and soil organic matter (SOM) sequestered by CDs and SSDs together with the sediment, and (iii) asses the climate change mitigation role of such sediment trapping structures.

Materials and methods

Study area description

The study was conducted in the Amhara National Regional State, northwest Ethiopia (Fig. 1), at eight sediment storage dams constructed at the outlet of micro-watersheds with an area ranging from 35 to 105 ha (Segno Gebeya, Woybila, Shehena Borkena, Enchet Kab, Tigrie Mender, Worka Wotu, Dodota, Wuha Chale) and six check dams constructed within gullies for the purpose of gully treatment (Rim/Debre Yakob, Minizr/Adibera, Gosheye, Debre Mewi, Debre Tabor and Bure). Farmland is the dominant land use type in each sub-catchment amounting to about 80% while about 20% is used as grazing land, eucalyptus plantation and/or bush land. The slopes in the sub-catchments ranged from 0.4–31% with dominant average slopes of 11.6–24%.

Fig. 1
figure 1

location map of the study areas

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During site selection difference in soil type, elevation and rainfall amount were considered. Five structures were selected at an elevation less than 2200 m a.s.l.; five structures between 2200 and 2700 m a.s.l. and four structures above 2700 m a.s.l. Difference in soil types were also taken into account such as Nitosols, Cambisols, Leptosols, Regosols and Vertisols. Table 1 summarizes the geographic location (X, Y coordinates), mean annual rainfall, soil type and elevation of each of the study sites.

Table 1 Study sites geographical location, soil type, rainfall and elevation
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Research methods

Measuring trapped sediment

In this study, eight sediment storage dams (SSDs) and six check dams (CDs) were selected and the amount of sediment trapped behind each structure was measured based on the geometric nature of the gullies, SSD/CD dimensions and the area of sediment deposition using GPS and measuring tape. Some of the structures have trapezoidal shapes and others have rectangular shapes. Figure 2 shows two example SSDs investigated.

Fig. 2
figure 2

Example SSD pictures constructed to sequester sediment and SOC two years old (left, Segno Gebeya watershed) and five years old (right, Woybila watershed), in the northwest highlands of Ethiopia

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To calculate the volume (V) of the sediment accumulated behind the trapezoidal shape SSDs/CDs, the area (A, m2) of the sedimentation times the length (L, m), which is from the SSDs/CDs to the end of sedimentation upstream was used (Eq.1). The area (A) of the trapped sediment is the average of the top and bottom widths (b1 and b2; m) of the sediment multiplied by its height (h, m) measured from the base of the dam to the sediment surface (Eq.2). For rectangular shape SSDs/CDs, length (L, m) times width (W, m) times depth (D, m) of the trapped sediment was used.

$$ V=A\ast L $$
(1)
$$ A=\frac{1}{2}\left(b1+b2\right)\ast h $$
(2)

Dry sediment mass calculation

To convert sediment volume, which was directly measured in the field to dry sediment mass, the bulk density of the trapped sediment was estimated using the cylindrical core method (McKenzie et al., 2002, Mekonnen et al. 2015). In the middle of the deposited sediment from 1 to 1.5 m deep pit (based on the depth of the deposited sediment) was dugout vertically downward and cylindrical core sampling was done at three locations along the side walls of the dugout pits (upper, middle and lower) inserting the cylindrical core sampler (100 cm3) into the side wall at the desired depth. The collected samples were oven dried at 105 °C in the laboratory for 24 h and sediment density was calculated weighing the dried sediment and subtracting it from the wet sediment mass.

Quantifying sediment organic carbon and organic matter

To quantify the amount of sediment organic carbon (SOC) and sediment organic matter (SOM) sequestered by SSDs and CDs together with the trapped sediment, from 1 to 1.5 m deep pits were dugout vertically downward in the middle of the deposited sediment behind the SSDs and CDs based on the depth of the trapped sediment. Sampling was done at three locations (upper, middle and lower) along the side of the walls in each pit. Samples collected from the upper, middle and lower locations were thoroughly mixed and 1 kilogram composite sediment sample was taken for laboratory analysis for each location, i.e. 14 kg composite sediment samples were used for laboratory analysis in total. The Walkley-Black titration method (Nelson and Sommers, 1982), which is one of the cheapest and rapid methods for the analysis of organic carbon (OC) in soils and sediments, was used. Soil organic matter was calculated by multiplying soil organic carbon by a factor of 1.724.

Results

Trapped sediment

The eight SSDs and six CDs investigated, built from gabion and stone trapped a total of ~ 50.5*103 m3 or ~ 68.8*103 t of sediment (44*103 m3 or 60.97*103 by SSDs, and 6.5*103 m3 or 7.8 t by CDs).

Sediment bulk density values ranged from 1.18–1.53 g cm− 3 with an average value of 1.36 g cm− 3 in the case of SSD sediments and from 1.04–1.32 g cm− 3 with an average value of 1.23 g cm− 3 in the case of CD sediments. Sediment bulk density is lower in heavy clay sediment deposits and higher in sandy loam dominated sediments. Sediment bulk density of the trapped sediment in SSDs and CDs ranged from 1.04–1.53 g cm− 3 with an average value of 1.3 g cm− 3. Table 2 shows volume, mass and bulk density of the trapped sediment behind each SSDs and CDs.

Table 2 Volume and mass of sediment trapped by SSDs and CDs
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Sediment organic carbon and organic matter

Through laboratory analysis, the deposited mass of SOC and SOM were determined (Table 3). SOC trapped by SSDs ranged from 14 to 87 g kg− 1 of sediment. SOM trapped by SSDs ranged from 24 to 147 g kg− 1 of sediment. SOC trapped by CDs ranged from 20 to 290 g kg− 1 of sediment. SOM trapped by CDs ranged from 35 to 530 g kg− 1 of sediment.

Table 3 SOC and OM trapped by SSDs and CDs
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The evaluated eight SSDs trapped ~ 28*105 kg SOC and 56.6*105 kg SOM together with 61*106 kg sediment and the six CDs trapped 16.68*105 kg SOC and 29.2*105 kg SOM together with 78*106 kg sediment. A kilogram of sediment contains 47–59% more SOM compared with SOC (Table 3 and Fig. 3). SOC and SOM contained in a kilogram of sediment showed direct correlation with R2 = 0.99 (Fig. 4). In general, the studied SSDs and CDs sequestered ~ 44.68*105 kg of SOC and 85.8*105 kg SOM together with ~ 68.8*106 kg of sediment.

Fig. 3
figure 3

Amount of SOC and SOM contained in a kilogram of sediment collected from fourteen locations

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Fig. 4
figure 4

the correlation between SOC and SOM contained in a kilogram of sediment collected from fourteen locations

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Discussion

Role of SSDs and CDs practices in trapping sediment

Rising soil erosion emphasises the need to trap sediment along the sediment transfer pathways. Dam construction of both large and small sizes to trap sediment can reduce soil erosion, downstream sedimentation, flooding and other environmental problems. Sediment storage dams (SSDs) and check dams (CDs) are soil and water conservation practices constructed in large areas of Ethiopia by Governmental offices and non-governmental organization with the objective to trap sediment and reduce soil erosion (Mekonnen et al. 2014; Mekonnen et al. 2015; MERET, 2008). SSDs are physical structures or barriers made of stone or gabion mostly constructed at the outlets of catchments and CDs are physical structures like SSDs but mostly constructed within gullies.

The sediment trapping role of eight SSDs and six CDs were investigated in this study and the result shows that SSDS trapped 44*103m3 or 60.97*103t and CDS trapped 6.5*103m3 or 7.8*103t of sediment. Both SSDs and CDs trapped a total of ~ 50.5*103m3 or ~ 68.8*103 t of sediment. Dams are important practices in reducing downstream sedimentation problem. In addition to reducing downstream reservoirs sedimentation, SSDs contributed in conserving soil within the larger catchment and re-filling and stabilizing gullies. An SSD constructed at Woybila catchment within a gully, which is serving as a temporary drainage channel during the rainy seasons, trapped ~ 22*103 t of sediment and refilled a 8 m deep and 20 m wide gully in 5 years reducing slope gradient by 12% on average, which can slow down the speed of runoff and give time for infiltration and sediment deposition. The sediment trapping role of dams of different sizes were evaluated by previous studies and proved their contribution. For example, Vorosmarty et al. (2003) noted that the world’s registered 45,000 large dams can trap 4–5 billion t yr.− 1 of sediment. In China more than 100,000 smaller check dams trapped 21 billion m3 of sediment (Wang et al., 2011). Sougnez et al. (2011) estimated the sediment volume trapped by 20 check dams in southern Spain as ranging from 4 to 920 m3. Sediment trapping dams not only help to trap sediment but also can be used to estimate sediment yield of the sediment contributing catchments above the dams, to refill gullies with sediment and reduce gully channel gradient.

Sediment a sink for organic carbon

Sediments trapped by sediment trapping dams or structures can potentially serve as carbon sinks (Poch et al. 2006; Harden et al. 2008; Van Oost et al. 2008; Cao et al. 2009, 2010; Zougmoré and Mando, 2010). Globally sedimentation resulting from soil erosion can sequester ~ 1 Pg C yr.− 1 (Stallard 1998; Smith et al. 2001). Bao (2008), Wang et al. (2011) and Li et al., (2007) evaluated the SOC sequestration role of CDs and the result showed that CDs trapped over 21*109 m3 with an average SOC content of ~ 3.31 g kg− 1 (Bao, 2008) and 0.952 Gt with an average SOC content of 3.4 g kg− 1 (Wang et al. 2011) and from 2 to 43 g kg− 1 of sediment (Li et al., 2007). In this study, SSDs and CDs showed a great role in sequestering carbon in the form of SOC. SSDs trapped ~ 60.97*103 t of sediment with the SOC content ranged from 14 to 87 g kg− 1 and CDs trapped 7.8*103 t of sediment with the SOC ranged from 20 to 290 g kg− 1. The SOC contents obtained within the trapped sediments are higher compared with other study findings mentioned above. The potential reason will be that the mass of SOC will vary spatially since the sources of carbon differ spatially. The possible sources of such SOC is decomposed crop stubbles, plant leaves and fertilizers applied in fields by farmers in addition to the nutrient content of the soil in the area. Moreover, the SOC content of sediments will increase when soil erosion is intensified and will decrease when soil erosion reduced. This means the quantity of SOC storage by sediments is controlled by the amount and type of organic residues that enter the soil (i.e. the input of organic C to the soil system) (FAO and ITPS, 2015).

SSDS and CDs also played an important role in trapping SOM. The SOM contents sediments trapped by SSDs and CDS ranged from 24 to 147 g kg− 1 and 35–531 g kg− 1 of sediment, respectively. The SOM content of sediments was higher than its SOC content. A kilogram of sediment contains 47–59% more SOM compared with SOC, which agreed well with the findings of FAO and ITPS, (2015) that is SOM contains roughly 55–60% C by mass. The mass SOC and SOM contained in a kilogram of sediment also showed direct correlation with R2 = 0.99. The evaluated eight SSDs trapped ~ 56.6*105 SOM together with 61*106 kg sediment and six CDs trapped 29.2*105 SOM together with 78*105 kg sediment. This shows that SSDs and CDs are also important C sinks. In general, the studied SSDs and CDs totally sequestered 85.8*105 kg SOM together with ~ 68.8*106 kg of sediment, which is a great contribution in reducing the GHGs concentration in the atmosphere. It also helps to have a good knowledge of the current SOC and SOM sinks, its spatial distribution and its sinking mechanisms to inform various stakeholders (e.g. farmers, policy makers, land users) to provide the best opportunities to mitigate climate change. After carbon enters the soil in the form of organic material from soil fauna and flora, it can persist in the soil for decades, centuries or even millennia. When the soil carbon is released into the atmosphere it becomes an important source GHGs. Soils are major carbon reservoirs/sinks containing more carbon than the atmosphere and terrestrial vegetation (Lal, 2003; FAO and ITPS, 2015; FAO, 2017).

Conclusion

Sediment storage dams (SSDs) and check dams (CDs) were found to be important structural sediment trapping measures trapping large amount of carbon at the outlets of small sized catchments and within gullies. The eight SSDs and six CDs investigated, trapped a total of ~ 50.5*103 m3 or ~ 68.8*103 t of sediment. In addition to trapping the sediment and reducing soil erosion SSDs and CDs played a promising role in sequestering soil organic carbon (SOC) and soil organic matter (SOM). The result shows that SSDs and CDs trapped from 14 to 87 and 20–290 g of SOC within a kilogram of sediment, respectively. In general, the studied SSDs and CDs sequestered ~ 44.68*105 kg of SOC together with ~ 68.8*106 kg of sediment. This means sediments are important reservoirs of SOC and plays an important role in reducing the amount of organic carbon that could be released to the atmosphere as a GHG. In conclusion, SSDs and CDs have retained substantial amount of carbon that could otherwise release to the atmosphere and contribute to global warming and thus SSDs and CDs can be used as climate change mitigation measures in additions to trapping sediments as soil and water conservation practices.